Computer disks and tapes and other media for storing electronically recorded information are especially vulnerable to damage by heat. In fact, temperatures well below the combustion temperature of paper may cause significant damage to electronic storage media. For example, computer disks are easily warped by heat, rendering them unreadable by computer disk drives. Electronic storage media also need to be protected against excessive humidity, which can cause similar damage.
Accordingly, fire-resistant enclosures of the kind normally used for protecting important papers do not meet the much more stringent requirements for protecting electronic storage media. Protection against the relatively small amounts of heat and humidity that can damage electronic storage media is provided by constructing fire-resistant enclosures for the media from several layers of specially selected materials. For example, U.S. Pat. No. 3,559,594 to Miller discloses a fire-resistant safe for protecting electronic storage media that includes: an inner repository made from a heat-conducting material, slabs of phase-changing material in contact with portions of the inner repository, a layer of heat-insulating material surrounding the inner repository and phase-change material, a layer of water-bearing material encasing the insulation, and an outer cabinet made of a metallic material enclosing the three layers that protect the inner repository. The safe of Miller may also include a sheet of moisture-resistant material forming a vapor barrier between the layer of water-bearing material and the layer of heat-insulating material. Together, the multiple layers of the Miller safe cooperate to protect storage media contained in the inner repository against excessive heat and humidity for a predetermined amount of time while the safe is subjected to an external temperature induced by fire.
U.S. Pat. No. 4,574,454 to Dyson discloses another fire-resistant enclosure for protecting electronic media that uses the same layers as Miller, but is intended to eliminate separate subassemblies which are said to be an apparent construction practice of earlier enclosure designs. Dyson complains that the practice of constructing the enclosures from separate subassemblies is time consuming and wasteful of material in the provision of separate casings for the subassemblies. The prior designs known to Dyson include a first subassembly that encases the water-bearing material between inner and outer steel skins. The presence of a steel casing intermediate the inner and outer skins is explained to aggravate a problem of heat "in-leak" from the exterior to the interior of the prior enclosures. Dyson proposes to solve these problems by eliminating the inner steel skin and building up all of the layers of protective material from the inside out, and this contrasts with Miller who builds up the same layers from outside in.
Nevertheless, both Dyson and Miller include the above-mentioned intermediate steel casings in the form of jambs for their respective doors. Dyson's enclosure also includes a wooden heat-break frame for insulating the inner repository from the steel door jamb. This practice of using a wooden frame to insulate against a steel door jamb is also apparent from a diskette safe disclosed by U.S. Pat. No. 4,373,450 to Miller et al. Although the wooden frames provide some protection against the direct conduction of heat between the steel door jambs and inner repositories of Dyson and Miller et al., the jambs conduct heat into the interior of their respective enclosures, thereby circumventing the important protections provided by the layer of water-bearing material.